1. Field of the Invention
The present invention relates to an image processing method including error diffusion processing, and an image processing device using this method.
2. Description of the Related Art
Conventionally, when performing color correction to improve color reproducibility of a color image between input and output devices, such color correction is accomplished by methods using a color look-up table describing the correspondence between the color of the input and output image at each color point within a color space, methods which determine a corresponding color using a color look-up table only for suitably quantified color points when there are many such color points and by interpolation from a certain color point in a color look-up table for other color points, and conversion by calculation method based on masking theory.
When the number of reproducible color in the output device is less than the number of colors of the input device, and when the amount of data are reduced for saving and transfer, a decrease of color depth is performed. In binary image output devices such as color displays and color printers which form each color point by mixing two primary colors, the input image is broken down to primary color components of yellow Y, magenta M, cyan C, or yellow Y, magenta M, cyan C, black Bk, or red R, green G, blue B, and the respective color components are subjected to stimulated gradient expression processing, then the obtained binary images are combined and output.
This type of image processing uses an error diffusion process to diffuse errors generated when processing a target pixel to peripheral pixels. The colorimetric value of each output color is dependent on the device. When halftones are expressed using this same output device, the colors will appear different if the degree of dot overlay differs even though the dot generation rate of each color CMYBk is the same. However, in general color error diffusion processing, a device-dependent input signal is used and since the overlay of dots of the same pixel is not considered, it is difficult to improve color reproducibility.
A method for producing halftones in color vector space by error diffusion process is disclosed in Japanese Laid-Open Patent No. 9-307776. This method treats input image data as vectors, i.e., multidimensional quantities, not as unidimensional quantities. Although it is possible to use device-dependent color signals as color signals treated as vectors, in the following halftone process the input and output colors can be expected theoretically to match calorimetrically using an input image expressed with uniform color space such as XYZ, CIELAB and the like which are not device dependent, and using the XYZ value and CIELAB value of the colors outputtable by the output device known beforehand. The outputtable colors of binary image output devices are the eight colors of cyan, magenta, yellow, red, green, blue, white, and black, with white color often using the calorimetric value of the paper itself.
(1) The input color vector and the outputtable color vector are compared, and the color having the smallest vector of vector difference of the two colors is selected. That is, the outputtable color nearest the input color on the image space is selected.
(2) The error between the input color and output color generated by the color selection is calculated.
(3) The generated error in a processed pixel on the periphery of an unprocessed pixel is used for weighted addition to correct the input color. Thereafter, the process (1) is performed for the unprocessed pixels.
This method is referred to as a vector error diffusion method, which makes possible high fidelity color reproduction using comparatively few colors, and readily corresponds to the outputtable color of halftones and spot color addition.
Conventional image processing devices using a vector error diffusion method have certain disadvantages arising from the paucity selectable output colors, including texture noise wherein a certain specific selection color appears periodically, and markedly reduces the printed image (particularly graininess) of the image.